This invention relates to color projection systems, and more particularly relates to such projection systems incorporating a single electro-optical light modulator.
Color projection display systems are known in which a white light source is separated into red, blue and green sub-beams for separate modulation by corresponding color components of an incoming display signal, and then the modulated sub-beams are recombined into a full color display for projection onto a viewing screen. Modulation of the sub-beams is commonly carried out using three separate electro-optical light modulators such as liquid crystal display (LCD) panels, one for each of the three sub-beams.
However, in one type of color projection system, described for example in U.S. Pat. No. 5,532,763, the three sub-beams are all modulated by a single LCD panel. This is accomplished by shaping the sub-beams into band-shaped cross-sections, and scrolling the bands sequentially across the LCD panel, while synchronously addressing those portions of the panel which are illuminated by the bands with the corresponding display signal information. Such projection systems are referred to herein as single panel scrolling raster (SPSR) projectors.
A disadvantage of all such SPSR systems is the requirement for a relatively large LCD (having at least an approximately 1.3 inch diagonal for the current lamp technology), in order to accept the light from the three separate primary color beams. Such large LCDs are expensive, and require the use of comparatively large, expensive optical components.
Another disadvantage of such SPSR systems is that scrolling of the beams across the panel is generally accomplished using either a single, relatively large, rotating glass prism, or three separate, smaller prisms, either of which alternatives results in a bulky arrangement, the latter having several moving parts.
It is an object of the invention to provide an improved SPSR color projection system.
It is another object of the invention to provide such an improved system in which the optical architecture is simple and compact.
It is another object of the invention to provide such an improved system having only one rotating element to accomplish scrolling.
It is yet another object of the invention to provide such an improved system in which the illumination beam is not separated into separate color beams prior to scrolling.
In accordance with the invention, a SPSR color display system includes a single, relatively small rotatable optical element with transmissive surface portions having different color transmission bands, and a single electro-optical light modulator.
In operation, the element rotates about its axis while broad spectrum (e.g., white) light from a source is incident on the proximal side of the transmissive surface, and is separated into color components by the surface portions which transmit the desired color bands. Light transmitted by the proximal side of the surface is focussed on to the distal side of the surface by a toroidal (e.g., cylindrical) lens mounted co-axially inside the hollow of the drum. The transmitted light from the distal side is focussed onto the light modulator panel. The color bands transmitted from the element are scrolled across the panel by virtue of its rotation.
Driver electronics synchronously address the panel with the corresponding color components of the display information during scrolling, whereby a full color image is produced for projection to a viewing screen.
The use of such a transmissive element as the scrolling means in a SPSR system eliminates the need to separate the white light into sub-beams prior to scrolling, and thereby enables a smaller (less than 1.3 inch) light modulator panel, and correspondingly smaller optical components. For example, such a transmissive element, e.g., a drum, can have a cross-section of 30 mm diameter or smaller. Thus, a simple and compact optical architecture is possible, leading to a compact projector with a small component count.
In accordance with a preferred embodiment, at least one opposing side of a transmissive filter segment of the surface of the element comprises a clear element or an air space, whereby the occurrence of a color artifact in the display image is eliminated. In accordance with another preferred embodiment of the invention, the rotatable optical element has a polyhedral surface, and the transmissive portions comprise dichroic filters having a flat surface.